Wire cut electrical discharge machining devices are known in which a continuous wire-shaped electrode, unwound from a wire supply source such as a bobbin or reel, is fed into a work tank or pan in which the workpiece is held and, after being used to generate electrical discharges in the space formed between the wire electrode and the workpiece -- the machining "gap" -- is taken up by a recovery roller. In devices of this type, the winding of the wire electrode on the recovery roller side can have undesirable effects on the wire electrode which is traveling through the gap side.
Wire cut electrical discharge machining devices having a container provided outside of the work tank with sufficient capacity to recover the spent wire electrode, and a mechanism for conveying the wire electrode from the gap to the container are known. Generally, such wire conveying mechanisms include a device mounted in proximity to the wire recovery container to pull the wire electrode, and a part for feeding the wire electrode from the gap to the wire pulling device, for example multiple feed rollers and pulleys, conveyor belts, or guide tubes into which a fluid is introduced, have been used for this purpose. In addition, a device which imparts curl to the wire may be provided between the wire pulling device and the container, so that the wire can be stored in an orderly manner in the container. Alternatively, a device for cutting the wire into small pieces may be provided. Such additional devices improve the storage capacity of the container.
Normally, the tension imparted to the wire electrode as it travels through the gap side is controlled by the joint action of a brake device, which is provided between the wire supply device and the gap, and which imparts an adjustable damping on the wire, and the wire pulling device. The wire pulling device performs a portion of the tension control function, so that failures along the travel path between the gap and the wire pulling device, such as wire straying from the travel path or wire jamming, may be transmitted to the wire electrode passing through the gap and cause poor machining accuracy. In particular, such failures can easily occur when automatically threading the leading end of a wire having a diameter less than, for example, about 0.1 mm, to pass through the initial hole in the workpiece and on to the pulling device.